The humidifier that let's you see when it's ready to refill. Oasis works by allowing water to evaporate through the fabric. Unless the cap is opened, outside air cannot get in to replace the evaporated water. The vacuum created by this process causes the fabric to shrink inward indicating that it's time for a refill with the included refilling syringe. The humidifier is held in place inside the body of the instrument by a suspension bar that keeps it horizontal.